The present invention relates to a print head and, more particularly, to a printing mechanism of a print head in a wire matrix printer.
The most popular of various wire matrix printers, the is a so-called serial printer in which characters, numerals and so forth are printed on a paper set on a platen, by a wire matrix print head which is moved in parallel with the platen. Roughly speaking, the wire matrix print head has seven or nine wires arranged in a predetermined pattern. In addition, the wires are guided such that they are arrayed in one or two rows at the end of the print head. Solenoids corresponding to respective wires are energized selectively to project the wires through movable parts such as armatures, thereby to form dots on the paper.
The wire matrix print head of the kind described is sorted into an attracting type and a "cancellation type" in dependence upon the method of activating the movable part. U.S. Pat. No. 4,004,673 provides an example of a so-called attracting type print head wherein solenoids are electrically driven to attract the armature driving the styli. U.S. Pat. Nos. 4,004,668 and 4,225,250 provide examples of the "cancellation type" wire matrix print head, wherein resilient materials are fixed at their one ends and are attracted and flexed by associated permanent magnets. In operation, the attracting force of the permanent magnet is cancelled by the magnetic flux produced by the solenoid, so that the strain energy stored in the resilient member is released to act on the wire styli thereby to print dots on the paper.
The cancellation type matrix print head is superior to the attracting type, in that the heat generation in the waiting condition is small and that a large attracting force can be obtained even by permanent magnets of a smaller size. The matrix print head of the cancellation type, therefore, is better suited for practical use.
In the print head of the cancellation type, the strain energy is stored by the deflection of the resilient member, and the printing wires are made to fly as the stored energy is released. Namely, the resiliency of the resilient member is directly used as the printing energy acting on the printing wire. This means that the quality of the print is largely affected by the factors such as the resilient force exerted by the resilient members, extent of non-uniformity of the resiliencies and so forth.
Various efforts have been made to attain higher printing speed by, for example, reducing rotational moment of the magnetic members as much as possible. More specifically, it is suggested to minimize the weights of the magnetic members, levers connected to the magnetic members, wires and so on, and to minimize the distance between the point of action of the wire and the fulcrum around which the wire is rotated.
In, for example, commonly assigned U.S. patent application Ser. No. 480,788, a print head is proposed which attempts to minimize the distance between the point of action and the fulcrum, thereby attaining a high printing speed. More specifically, this improved print head has a plurality of magnetic members with one end of the magnetic member having wires are fixed directly or indirectly thereto, and permanent magnets carrying yokes for attracting the magnetic members, wherein the magnetic members are held in contact at their rear ends with the magnetic pole surfaces of the permanent magnets or the yokes, thus providing the fulcrums. According to this arrangement, it is possible to remarkably reduce the distance between the point of action of the wire and the fulcrum of the same and, hence, to achieve a high printing speed, as compared with known print heads.
Accordingly, an object of the invention is to provide an improved print head which attains higher wear resistance of at least the magnetic pole surface which serves as an attracting surface for attracting the magnetic member acting on the printing element and which cooperates with the magnetic member in defining therebetween a magnetic path.
It has been determined that, if the contact portion for contact with the magnetic member and serving as the fulcrum of rotation of the same, is formed on the magnetic pole surface of the permanent magnet or the yoke mounted on the permanent magnet in the cancellation type print head mentioned before, the member presenting the magnetic pole surface is worn rapidly.
According to the invention, at least the magnetic pole surface of the yoke or the permanent magnet presenting the magnetic pole surface is hardened by a treatment with magnetically permeable material. The hardening treatment is conducted, for example, by plating with a material consisting essentially of nickel. As another measure for hardening, a magnetically permeable sheet, subjected to a hardening treatment is formed on the magnetic pole surface, whereby taking a hard layer is formed on the magnetic pole surface to improve the wear resistance of the latter.
The hardening of the magnetic pole surface, however, may cause a rapid wear of the contact portion of the magnetic member, because the hardness of the contact portion is less than that of the magnetic pole surface. The invention, therefore, does not exclude the hardening treatment on at least the contact portion of the magnetic member.
Thus, another object of the invention is to provide a print head improved to attain a higher wear resistance of the magnetic member which actuates the printing element.
After the impacting of the printing element, the electric power supply to the solenoid is interrupted so that the magnetic member is reset by the attracting force of the permanent magnet and is stopped or collides with the core of the solenoid. This means that rapid wear tends to be caused also in the magnetic pole surface of the core.
Accordingly, another object of the invention is to provide a print head in which the magnetic pole surface of the core and at least the portion of the magnetic member facing the core are hardened by a treatment similar to that explained before to exhibit greater resistance to wear.
In the print head of the type described, the amount of resilient deformation of the leaf spring is determined by the position and posture of the rotatable magnetic number serving as the armature. The position and posture of the magnetic member, however, are changed undesirably due to wear of the magnetic member itself and the wear of the magnetic pole surface constituting the fulcrum of rotation of the magnetic member. In addition, different springs exhibit different amounts of resilient deformation. For these reasons, it is difficult to store constant strain energy in each spring and to uniformalize the energy in all springs. Consequently, there is a failure in a uniformity of operation of the armatures thereby impairing the quality of the printing.
According to the invention, since the suitable portions of the magnetic members, yokes and cores which may tend to show rapid wear are treated to exhibit higher resistance to wear, the lack of uniformity of the armature operation due to wear of such portions can be avoided to ensure a higher quality of the printing.
These and other objects, features and advantages of the invention will become clear from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.